Not since James Knox Polk, who kept a meticulous diaryhave Americans known a president as seemingly self-revealing as Richard M. Nixon, author of five books, mostly about foreign policy, since he published his memoirs in 1978. Nor has Mr. Nixon lacked for serious biographers, as those who enjoyed the efforts of Roger Morris and Stephen E. Ambrose can attest.

Curiosity about the brooding loner from the lemon groves of Yorba Linda, Calif., persists today, 17 years after he resigned in disgrace. For those who loved him and those who did not, Tom Wicker, columnist for the New York Times, has channeled his obsession into an admirable biography.

This summation of Mr. Nixon from a liberal critic fulfills the wish that Mr. Nixon asked from the press in his famous "last press conference" in 1962 after losing the governorship of California. Mr. Wicker gives Mr. Nixon "a fair shake." He has cataloged many of Mr. Nixon's considerable accomplishments in domestic and foreign policy, all obscured in historical perspective by Watergate, toward which the author is too forgiving.

On the emotional issue of Vietnam, Mr. Wicker challenges critics of Mr. Nixon's "silent majority" speech in which he said, "A nation cannot remain great if it betrays its allies and lets down its friends." Mr. Nixon, who invested most of his sincerity in a strategic American Realpolitik, "is often scoffed at as a man of no principle, motivated only by political advantage. On the question of continuing the war, however, critics can't have it both ways -- that he refused to withdraw and had no principles. Had the latter been true, he almost surely would have abandoned the war in his first months in office."

Mr. Nixon's place in history may await the shrinking of both Watergate and Vietnam in historical perspective. In chapters called "Reformer" and "Keynesian," Mr. Wicker details how Mr. Nixon, with the advice of Daniel Patrick Moynihan, empathized with and fought for the poor more than any succeeding president. Had Democrats in Congress accepted Mr. Nixon's Family Assistance Plan 20 years ago, America's children would be better off. Just as he boldly sought detente with China and the Soviet Union, so Mr. Nixon was unafraid to innovate domestically and to defy ideology, as he did in freezing wages and prices in 1971.

Why did he do it? Motivation, Mr. Wicker writes, is "a central question about Richard Nixon. Did he want to do great things, or only to get credit for doing them? Or perhaps the true question is whether that really matters, for Mr. Nixon or anyone, assuming the great things get done -- or, if they don't, that a genuine effort has been made. Does purity of motive confer some finer quality upon achievement? If the urge to do good is really the desire for credit, or if the urge is stimulated by the desire, is the good that may be done diminished?"

The former president did not talk to Mr. Wicker for this biography, forcing the author to rely upon Mr. Nixon's published works as evidence. These writings, alas, damage Mr. Nixon's case because as a memoirist he is less persuasive than as a politician. The self-serving cant of Nixon prose is evident in Wicker's first citation from "Six Crises": "I knew that what was most important was that I must be myself." Mr. Nixon regarded sincerity as an option, and his version of events do not always jibe with that of others.

When "One of Us" wanders away from its main subject and sketches events of Mr. Nixon's time, the book becomes less Nixon-obsessive-appealing and more New York Times-dutiful. Mr. Nixon is like a dominating screen presence, an amalgam of leading man and character actor -- Lee Marvin, Sean Connery, even John Wayne -- whose absence from the action tends to make the audience doze off. An exposition of Mr. Nixon's possible drinking problem, for instance, is far more interesting for two pages than an account of the 1968 presidential election is for 20.

Those sympathizing with such an inferiority complex might beware of its flip side, a superiority complex. Mr. Wicker does not engage in psycho-history but this work, which Marxists call "revisionist," forgives Watergate more readily than credibility demands. It is especially distressing that Mr. Wicker paraphrases Lord Acton's dictum of power to do so: "Because power corrupts, and because the real and effective power of an American President had been so greatly expanded, by the time Richard Nixon reached the White House, Watergate or something equally disreputable was a disaster waiting to happen."

Is Richard Nixon a victim of history? Lord Acton would not think so. Leaders must be denied "a favorable presumption that they did no wrong," he wrote in 1887. "Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt and absolute power corrupts absolutely."

Would "one of us" treat an oath to preserve, protect and defend the Constitution so lightly? Mr. Wicker defames Americans to defend his subject. Mr. Nixon deserves a better shake from history and Wicker helps him get it, but it is fruitless to deny that he was seduced by power. Acton had the papacy in mind when he wrote, "There is no worse heresy than that the office sanctifies the holder of it."

That was Richard Nixon's heresy. He did many admirable things but failed when he acted as though he and his holy office, "the presidency," were above the law that he had sworn to uphold.